WASHINGTON — With the U.S. Senate having approved major health care reform legislation Thursday during an extraordinary Christmas Eve vote, Barack Obama moved one step closer to reshaping American medicine and accomplishing that which has eluded previous Democratic presidents.

But looming as Obama’s final obstacle to signing a bill into law are negotiations to reconcile the $871 billion Senate legislation with a $1.2 trillion health care package passed by the House of Representatives back in mid November. Whether through a formal conference committee of several Senatorsand Congressmen, or driven from the top down by Senate Majority Leader Harry Reid (Nev.) and House Speaker Nancy Pelosi (Calif.) — Democratic leaders haven’t decided which, merging the two bills could prove tricky.

Just one Republican voted for the House bill; and in the Senate, the GOP was unified in its opposition.

Accordingly, the path of least resistance for Obama would suggest the House swallow the Senate bill and not risk fracturing the coalition of 60 Democratic Senators that were needed to overcome a Republican filibuster of that chamber’s legislation. But House Democrats are not necessarily inclined to roll over and play dead, potentially creating a headache for the president as he seeks to put this deal to bed before the 2010 elections overtake lawmakers’ focus.

As House Rules Committee Chairwoman Louise Slaughter (N.Y.) wrote in an op-ed on CNN.com, “Although the art of legislating involves compromise, I believe the Senate went off the rails when it agreed with the Obama Administration to water down the reform bill . . . It’s time that we draw the line on this weak bill and ask the Senate to go back to the drawing board.” Congress is taking a break between Christmas and New Year’s. But in early January, Democratic leaders and the White House are set to begin seriously discussing how to reconcile the House and Senate health care bills. With the finish line so near, things just might go smoothly. But if the two chambers lock horns, here are the issues that could be at the center of the fracas:

* The government-run insurance option. Liberal Democrats in the House, led by Pelosi, strongly favor a government-run insurance option, and it is included in the House bill. The measure is among the most sought after health reform goals of those on the left, and some Democrats believe a bill without it is not worth supporting. But the proposal was dropped from the Senate version after Sen. Joe Lieberman, a Connecticut Independent who caucuses with the Democrats, vowed to filibuster. Other moderates in the Senate Democratic caucus also oppose the public option, and including it in the final bill could blow up their 60-vote coalition.

* Abortion. At the insistence of pro-life Democratic Rep. Bart Stupak (Mich.) and like-minded party members, the House bill includes unequivocal language prohibiting federal funds from being used to pay for abortion procedures. The Senate bill includes a weaker provision inserted as a compromise after prochoice Democratic Senators helped kill a proposal similar to the House measure. If pro-choice Democrats in the House and Senate move to delete the abortion language altogether, it could cost crucial votes in both chambers. Strengthening the abortion language in the Senate bill could cost Democratic votes there; weakening the House language could cost Democratic votes there.

* Taxes. The House bill proposes to implement a new 5.4 percent income surtax on individuals earning $500,000 or more annually and couples earning $1 million. Democratic Senators have indicated that provision is a nonstarter. The Senate, meanwhile, proposes taxing expensive health insurance plans. Any individual policy worth $8,500 or more annually, and any family policy worth $23,000, would be subject to a new 40 percent excise tax. Although this tax would be levied on the insurance companies that issue “Cadillac” plans, critics argue that it would be passed along to consumers. More troublesome for House and Senate negotiators, the labor unions — an important Democratic constituency — hate this proposal, making it unclear if House Democrats will accept this provision.

* The Senate bill includes many special deals seen as crucial to maintaining the support of certain Senators for a final, reconciled bill. But a negative a public outcry over those special agreements could create a groundswell to have those provisions nixed or adjusted. Additionally, House Democrats could seek to add their own constituent perks, potentially making it harder to meet Obama’s demand for a bill costing no more than $900 billion over 10 years without adding to the deficit.

* Sen. Ben Nelson’s (D-Neb.) deal to have the federal government fully fund the Medicaid expansion has received the most attention of late. But Sen. Mary Landrieu (D-La.) extracted $300 million in Medicaid funding for her state; Sen. Chris Dodd (D-Conn.) secured a $100 million earmark for his state; Sen. Bernie Sanders, a Vermont Independent who caucuses with the Democrats, received extra Medicaid money for his state; and Montana, North Dakota and South Dakota, among others, received extra Medicare money.